TISSUES OF THE STEM 341 



such as Pteris, the stem ends in a blunt, knob-like extremity 

 quite uncovered by leaves. On the proximal portion of the 

 stem are usually found the withered remains of the leaves 

 of previous seasons, or the scars left by their fall. The 

 roots are given off from the whole surface of the stem, 

 often covering it with a closely-matted mass of dark brown 

 fibres. 



When the stem is cut across transversely (Fig. 83, A) it 

 is seen, even with the naked eye, to consist of three well- 

 marked tissues. The main mass of it is formed of a w r hitish 

 substance, soft and rather sticky to the touch, and called 

 ground-parenchyma (par) : this is covered by an external 

 layer of very hard tissue, dark brown or black in colour, the 

 hypodermis (hyp) : bands of a similar hard brown substance 

 are variously distributed through the parenchyma, and con- 

 stitute the sdereHchyma (set] : and interspersed with these 

 are rounded or oval patches of a yellowish colour (V.B) 

 harder than the parenchyma, but not so hard as the 

 sclerenchyma, and called vascular bundles, 



The general distribution of these tissues can be made out 



O 



by making longitudinal sections of the stem in various 

 planes or by cutting aw r ay the hypodermis, and then scraping 

 the parenchyma from the vascular bundles and bands of 

 sclerenchyma. The hypodermis is found to form a more or 

 less complete hard sheath or shell to the stem, while the 

 internal sclerenchyma and vascular bundles form longi- 

 tudinal bands and rods imbedded in the parenchyma, and 

 serve as a sort of supporting framework or skeleton. 



The minute structure of the stem can be made out by 

 the examination either of very thin longitudinal and trans- 

 verse sections, or of a bit of stem which has been reduced 

 to a pulp by boiling in nitric acid with the addition of a few 

 crystals of potassium chlorate : by this process the various 



